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How it Works: Water for Coal

Coal-fired power plants, which produce almost half of the country’s electricity, have significant impacts on water quantity and quality in the United States. Water is used to extract, wash, and sometimes transport the coal; to cool the steam used to make electricity in the power plant; and to control pollution from the plant. The acts of mining and burning coal, as well as dealing with the waste, also can have major effects on water quality.

Electricity Generation

Like all thermoelectric power systems, coal plants require cooling. Three major options are available: once-through, wet-recirculating, and dry cooling. About 53 percent of coal plants in the United States use once-through cooling, about 40 percent use wet-recirculating, and less than one percent use dry-cooling.[1] Table 1 shows water requirements in gallons per megawatt-hour (MWh, or thousand kilowatt-hours) of electricity production. (Despite their name, dry-cooling systems still require water for system maintenance, cleaning, and blowdown, as explained below).[2]

Table 1: Water requirements for cooling by type in gallons per megawatt-hour for conventional coal power plants[3]

The choice of cooling system used in a coal plant affects not only its water requirements but also the efficiency of the power plant as a whole. According to estimates by the U.S. Environmental Protection Agency (EPA), coal plants that use dry cooling produce about seven percent less power than those that use wet-recirculating systems.[4] Because coal power derives all of its energy from producing steam, dry cooling has a greater impact on the efficiencies of coal-fired plants than on most natural gas-fired ones. [5],[6]

Coal boilers also use small amounts of water for boiler blowdown. In this process, water is bled from the boiler to get rid of impurities that accumulate and form sludge that can impair a plant’s performance.

A more efficient coal technology, called integrated gasification combined-cycle (IGCC), is being commercialized. Along with reducing air pollutants, this process can decrease water consumption by 35-60 percent compared to conventional coal plants.[7]

Coal Mining and Transport

Coal can be mined from deep underground caverns, surface pits or from mountain tops. Mountaintop removal, the most destructive mining method, also has tremendous water impacts. In this method of extraction, mining companies use explosives to remove the top layers of rock and dirt above a coal seam and discard the resulting debris, usually in an adjacent valley. This technique can bury streams, contaminate local water sources, and increase the risk of flooding. EPA estimates that strip mining of coal by mountaintop removal has buried almost 2,000 miles of Appalachian headwater streams, some of the most biologically diverse streams in the country.[8]

Depending on its quality, coal may need to be “washed” with water and chemicals to remove sulfur and impurities before it can be burned in a power plant. According to the U.S. Department of Energy, total water used for coal mining in the United States (including water use for coal washing and cooling of drilling equipment) ranges from 70 million to 260 million gallons a day.[9] Storing coal-mining waste together with the water used to separate it from the coal can present a significant hazard if the impoundments (“slurry ponds”) fail or the slurry breaks through into nearby abandoned mines.

After extraction, coal must be transported to the power plant. While most U.S. coal travels by train, barge, or truck, some travels by the slurry pipeline method, which involves pumping water with finely ground coal over long distances. Slurry pipelines withdraw hundreds of gallons of water for every megawatt-hour of electricity produced.[10]

Post-Combustion

Burning coal emits large quantities of pollutants, including sulfur dioxide, carbon dioxide, nitrous oxides, and mercury. Sulfur dioxide and nitrous oxides can mix with rain or snow to form acid rain. This mixture increases the acidity of lakes and streams and can harm or kill plants and animals. Mercury is a potent neurotoxin that reduces intelligence and otherwise impairs the brain development of infants and children, and that has been linked to heart problems. According to the U.S. EPA, coal plants are the source of over half of anthropogenic (human-caused) emissions of mercury to the air in the U.S.[11] After leaving the smokestack, the mercury falls to earth and accumulates in water bodies and subsequently in the tissues of fish and of people and animals that consume those fish.

Pollution control equipment on power plants, called “scrubbers," can reduce the emissions of sulfur dioxide to the atmosphere by using a mixture of limestone and water to absorb pollutants. This process produces close to 200,000 tons of sludge waste per year for a typical power plant.[12]

Coal ash is another substance with water implications that coal power plants emit in large quantities. Sludge and coal ash wastes are often disposed of in unlined landfills and reservoirs. Heavy metals and toxic substances contained in this waste can contaminate drinking water supplies and harm local ecosystems. When the coal ash waste dike associated with the Tennessee Valley Authority’s 1,500-megawatt Kingston Fossil Plant in Tennessee gave way on December 22, 2008, for example, it dumped an estimated 1.1 billion gallons of coal ash mixed with water into the Emory River.[13]

Early technologies for capturing and storing carbon emissions – while of interest for controlling heat-trapping emissions – can have substantial impacts on power plant efficiency and require large volumes of additional water. Adding carbon capture and storage to a coal plant would increase its water consumption 45 percent to 85 percent.

[5] Most natural gas plants–combined-cycle and simple combustion turbines–generate electricity at least in part using the force of the gases from the combustion, rather than relying solely on a steam cycle, therefore requiring less cooling for the same electricity output.

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